Limits of sustaining a flame above smoldering woody biomass

ABSTRACT

Smoldering is slow, low-temperature, and flameless burning of porous fuel, and it is also an emerging method for energy conversion and waste removal. However, smoldering combustion is an incomplete combustion process, so the pollutions resulted from smoldering emissions are significant concerns. This work explores the flaming of emission gases from the smoldering wood chips (200 kg/m³) under different oxidizer flow velocities (4 mm/s-24 mm/s) and oxygen concentrations (14%-21%) through porous media. Once ignited on the top, the smoldering front first propagates downward (1^st stage, opposed) to the bottom and then propagates upward (2^nd stage, forward). We found that during the 1^st-stage downward smoldering propagation, a stable flame of smoldering emissions could be piloted and sustained. The critical smoldering burning rate for maintaining a stable flame remains constant at 10–12 g/m²∙s. To reach such a minimum smoldering burning rate, the required opposed flow velocity increases from 6 to 24 mm/s, as the oxygen concentration decreases from 21 to 14%. A simplified heat transfer process is proposed to reveal the limiting conditions for the co-existence of flaming and smoldering. This work enriches strategies for the clean treatment of smoldering emissions and promotes an energy-efficient and environment-friendly method for biowaste removal.

KEYWORDS:

• Smoldering combustion
• flammability limit
• fire spread
• porous media
• oxygen concentration

Nomenclature

Table

Acknowledgments

Authors thanks ECO-Greentech Ltd. for providing fuel samples, and Prof. Jose Torero (University College London) for supporting the research idea and funding application.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed on the publisher’s website

Additional information

Funding

This work is funded by National Natural Science Foundation of China [NSFC grant No. 51876183], ZJU SKLCEU Open Fund [2018012], Sichuan Science and Technology Program [2019YFSY0040], and Society of Fire Protection Engineers (SFPE) Educational & Scientific Foundation.